Materials used in grinding media applications where wear resistance is required include alloy steels, both cast and forged, and alloy white cast irons in the as-cast and heat treated condition. Alloy white cast irons are divided into two groups, one containing approximately 8 to 30% chromium and the other 0 to 4% chromium. High chromium white cast irons necessitate a special heat treatment above the transformation temperature to allow them to reach their full potential. This heat treatment combined with their high alloy content renders them uneconomical for many ore grinding applications. The other group of white cast irons is generally more economical although an amount of certain alloying elements is required to yield optimum properties.
It is widely recognized that low alloy white cast irons with a microstructure consisting of carbide and martensite offer high wear resistance in most ore grinding applications. Carbide is present in all white cast irons; martensite, on the other hand, is obtained through a combination of alloy content and processing conditions. If the conditions for martensite formation are not met, an undesirable phase known as pearlite will be produced upon cooling from the transformation temperature. Many alloying elements have been used over the years to avoid the formation of pearlite. These elements are used either alone or in the combined form, and they include nickel, chromium, molybdenum, copper, manganese and vanadium.
Because of its outstanding wear resistance, a nickel-chromium white cast iron known as Ni-Hard has been successfully used for over forty years to make grinding media. The microstructure of Ni-Hard consists essentially or carbide and martensite, and its hardness is 600 Brinell in the chill cast and stress relieved condition. Compositional ranges suggested for the manufacture of Ni-Hard grinding balls and slugs are about 3% carbon, 0.5% silicon, 0.5% manganese, 1.5-4% nickel and 1.0-2% chromium, the rest being iron.
A low alloy white cast iron having a hardness and a microstructure similar to those of Ni-hard was disclosed in Canadian Pat. No. 1,125,056 owned by the present assignee. The alloying elements were 2.5 to 4% carbon, 0.3 to 0.8% silicon, 0.3 to 1.0% manganese and 1.7 to 3.5% nickel.
The procedure followed for producing the above alloy was as follow: Iron and the above alloying elements were melted in a suitable furnace and cast into moulds. The cast product was shaken out of the moulds at a temperature above the transformation temperature and was either cooled with fine water sprays or subjected to forced air or still air cooling to cool it between approximately 1400.degree. F. and 400.degree. F. at a minimum of 2.5.degree. F./sec. The above combination of alloy content and processing conditions produced an alloy with a microstructure consisting essentially of martensite and carbide with no pearlite. However, the minimum amount of nickel needed in the above process to avoid the formation of pearlite was 1.7%. In addition, the hardness of the cast product was not uniform throughout.
Although the above low alloy white cast iron was less expensive than Ni-Hard because it contained less nickel and no chronium, it was felt that more research was needed to further decrease the nickel content and thus, to develop a more cost-effective alloy for grinding media application. It was also desirable to develop a product having a more uniform hardness.